In a heat exchange apparatus, tubes are in fluid communication with each other and used to convey a fluid through an apparatus for purposes of either heating or cooling the conveyed fluid. In certain applications, it may be desirable to join one or more tubes positioned in a relatively extreme thermal region of the apparatus to one or more tubes in a relatively less extreme thermal region either within or external to the apparatus, such that thermal expansion and/or contraction may move or impose relatively severe stresses upon, the joint between the tubes. For example, in a hydrocarbon steam cracking furnace, tubes are arranged to pass a feed from outside a firebox, into and through the firebox, and then out of the firebox to a quench system. Due to the temperature differentials involved in the process and potentially the internal pressure of the feed within the tubes, the tubes experience thermal expansion and thermal stresses. Accordingly, the tubes are arranged to minimize the thermal stresses wherever possible. The joint connecting the tubes may be particularly vulnerable to stresses. As such, it may be challenging to sustain desired performance properties. Special precautions, with respect to joint integrity, strength, and/or protection against inadvertent ruptures or leaks may be required. Such tubular connections or joints may also involve a special connection apparatus, such as a special union fitting, or include redundant connection mechanisms such as threads and welds. Where a leak may be particularly hazardous, such as posing a risk of fire or toxic release, it may be desirable to position the joint within the confines of the apparatus casing so that any potential leak may be consumed, oxidized, or pyrolyzed within the high heat zone (e.g., firebox or radiant heating section).
One exemplary heat exchange apparatus is a hydrocarbon steam cracking furnace. Steam-cracking pyrolysis is the predominant process used in the production of ethylene and propylene. Steam cracking is typically conducted in a direct-fired tubular reactor. Volatile hydrocarbon feeds, such as ethane, naphthas, gas-oils, and crude fractions, are heated rapidly in the presence of steam to produce ethylene, propylene, and other product species. The steam cracking process typically involves conveying a feed fluid from a lower temperature region (outside the furnace) into and through a high temperature region (inside the furnace), and then out of the high temperature region into another lower temperature region (outside the furnace).
One of the process variables in this process is reactor residence time. For example, a greater portion of the hydrocarbon feed is converted to ethylene and propylene if the reaction time in the radiant coil (e.g., tube, tubes, or tubulars) is kept as short as possible. The industry-standard measure of residence time is the time required to pass through the tube in the radiant firebox and the exit tube to the quench unit. One common approach to reducing residence time is by using high strength steel that can tolerate higher heat fluxes, thus enabling use of reduced coil length and surface area. Exemplary materials include wrought or cast high-alloy austenitic stainless steels. Another common approach is to use furnace coil designs of shorter and shorter lengths. To avoid exceeding the radiant tube heat flux and/or temperature limits, as the coil length is reduced, a larger number of smaller diameter coils are used in parallel because smaller diameter coils have a larger surface area-to-volume ratio than larger diameter tubes.
Metallurgical-related limitations are impediments to still further reductions in furnace residence times. Austenitic steels commonly used for the radiant coil and unfired adiabatic zone of ethylene furnaces have reasonable high temperature strength up to 1100-1150° C. and are weldable, using well-proven techniques. However, the austenitic steels have the disadvantage that the interior surface of the tubes and fittings catalyzes carbon deposits (e.g., coke). As this coke layer grows, it insulates the process gas, which results in tube wall temperature increases to provide sufficient heat for the reaction to proceed. Eventually, the tube wall temperature limit is reached and the furnace is taken out of service for decoking. Thus, while a given material may be capable of operating at temperatures, for example, of up to 1100° C., the furnace design is limited to a “clean-coil” tube wall temperature of about 1020° C. to allow for the temperature rise. This limitation constrains both how short a residence time may be designed in a coil and how much feed can be processed in a coil of a given geometry.
While there are other contributors to coke formation, tube surface catalyzed coking is generally accepted as a predominant cause of coke formation, particularly for light gas feeds such as ethane. Surface catalyzed coking rates increase as surface temperature increases. Thus, the temperature limits of austenitic stainless steel radiant tube materials and the coke that forms, at least in part, due to the catalytic reactions that take place at the tube surface effectively combine to prevent designers from reducing residence times below approximately 0.10 seconds. This also sets a limitation on the maximum ethylene yield that can be achieved from a furnace.
If the coking phenomena could be eliminated or substantially reduced, furnaces could be designed for higher “start-of-run” or “clean wall” tube metal temperatures, thus permitting shorter reaction times and thus higher product yields, and may also permit a higher feed rate to be processed through a given radiant coil design. Such an improvement may be realized by using radiant tubes manufactured from a material whose inner surface does not catalyze coke. In addition, if radiant tube materials are used which operate at higher temperatures than the current austenitic stainless steels, then coil length and residence time, may be reduced even more, leading to even further increases in ethylene yield.
To reduce the formation of coke, ceramic tubes, which resist formation of surface-catalyzed coke, may be utilized. However, ceramic materials and some metals that form alumina rather than chromia oxide layers have extremely low ductility and are brittle and, thus are susceptible to cracks and leaking. Further, they do not tolerate extreme rapid cooling without a tendency to crack or shatter. As such, use of these materials has only been acceptable inside the radiant firebox of the furnace where any leak is contained and oxidized within the refractory lined casing plate. Such use is unacceptable outside the casing plate of the radiant section of the furnace. Any leak outside the radiant section (either radiant inlet or outlet) could immediately result in a fire as the hydrocarbon material is above its auto-ignition temperature at these locations. Therefore, the radiant inlets and radiant outlets located outside the radiant firebox are constructed of more ductile materials such as the existing high-alloy austenitic stainless steels to minimize leakage and fire risks.
Joining low coke catalyzing radiant coils to the austenitic inlets and outlets is a major challenge. Welding alumina formers to austenitic stainless steels, while possible, often results in a weld joint of significantly reduced high-temperature stress-rupture strength than the strength of austenitic materials. Such joints may not have sufficient strength to tolerate the high temperatures experienced inside radiant sections of pyrolysis furnaces. For this reason, such joints may use a complex threaded joint for mechanical strength with a seal weld for gas tightness. Joining ceramics to austenitic stainless steels also poses great engineering challenges due to the significantly different co-efficients of thermal expansion. This leads again to the use of complex mechanical joints or low strength brazing techniques that are not well suited for the high temperatures of a radiant section.
In one aspect, what is needed is a heat exchange apparatus that facilitates maintaining sensitive tubular joint connections within the high heat zone casing of the apparatus, while also protecting the connection from any high or extreme temperature that may be encountered at operating conditions that may compromise such connection joint.
What is also needed is a hydrocarbon steam cracking furnace that incorporates radiant tubes made of materials that do not catalyze coke formation securely and safely joined to austenitic stainless steel inlet and/or outlet sections outside the radiant firebox.